A power semiconductor module composed of an IGBT power device, which performs switching control of a large current, and a diode, which releases an inverse voltage generated at the time of switching, is used in a wide range of fields such as home electric appliances and vehicles as a principal component of a power converter (inverter). Especially, electrically-powered vehicles are promoted and an environment in which the power semiconductor module is used becomes severe, so that this is used under a high-temperature atmosphere in which cooling cannot be sufficiently performed, and current capacity to be controlled is increasing. Therefore, as performance of the power semiconductor module, high reliability for securing normal operation for a long period of time in a usage environment with a large temperature change and a high heat resistance property to resist high temperature of a device due to an increase in heat generation amount from a device associated with large current energization are required.
In a conventional power semiconductor module, an insulating circuit substrate of ceramic and the like is joined onto a base substrate having high thermal conductivity formed of a Cu-based or Al-based material by soldering and the like, and an electrode surface of a semiconductor device is joined onto Al or Cu circuit wiring forming the insulating circuit substrate by means of soldering. Further, Al or Cu wire wiring is connected to a main electrode terminal on an electrode surface opposite to the surface, which is in contact with a circuit surface, of the semiconductor device (PTL 1, for example). In a conventional structure, a thermal stress applied to a joint part between the semiconductor device and the wiring or between the base substrate and the insulating circuit substrate increases along with the increase in heat generation amount from the device and this generates a crack in a solder layer to break the joint part. A thermal expansion coefficient of the base substrate and a wiring material is five to eight times larger than that of the semiconductor device and the ceramic insulating substrate, so that the stress and a strain of the joint part increase to cause the break of the joint part. Especially, a shearing stress applied on the same plane as a joint interface has a major effect on the break of the joint part. In order to decrease the shearing stress applied on the same plane as the joint interface, it is required to decrease the thermal expansion coefficient of the base substrate and the wiring material to a level comparable to the thermal expansion coefficient of the semiconductor device and the ceramic insulating substrate at least in a joint region.
As the wiring, the wiring material obtained by mixing a nano power or a nano fiber having a low thermal expansion coefficient with Cu is applied in PTL 1 and the wiring material obtained by impregnating a fiber having a low thermal expansion coefficient with Cu is applied in PTL 2. They are obtained by combination of a material having a low thermal expansion coefficient with an entire conductive material such as Cu. There are C, W, Mo and the like as the material having the low thermal expansion coefficient; electrical conductivity of such materials having the low thermal expansion coefficient is ½ or lower of that of Cu, Al and the like conventionally used as the wiring material. Although a specific content rate of the material having the low thermal expansion coefficient is not disclosed in PTLs 1 and 2, it is easily considered that the combination of the material having the low thermal expansion coefficient makes the electrical conductivity lower than that of the conventional wiring material to cause an increase in loss of current.